Various researches have been conducted on solid polymer-type fuel cells because solid polymer fuel cells can achieve reduced weight, high power density, etc., compared to other fuel cells. The solid polymer fuel cell comprises an ionic conductive polymer electrolyte membrane as its electrolytic membrane, a catalytic layer and an electrode substrate disposed on both surfaces of the electrolytic membrane in this order, and separators sandwiching the resulting laminate.
A solid polymer fuel cell ordinarily uses a cation conductive polymer electrolytic membrane, which transmits cations (H+). Generally, a perfluorosulfonic acid resin that has a main chain comprising fluorinated alkylene and a side chain comprising fluorinated vinyl ether having a sulfonate group at its end is used. Such a polymer electrolytic membrane conducts sufficient ions for power generation by being impregnated with a suitable amount of water.
Therefore, in known solid polymer fuel cells, the moisture content in the polymer electrolytic membrane must be controlled, and this inevitably results in complicated and large-sized fuel cell systems.
In order to avoid the problems attributable to controlling the moisture content in the polymer electrolytic membrane, the use of an unhumidified electrolytic membrane, which allows the conduction of protons in an unhumidified condition, in place of known polymer electrolytic membranes has been proposed.
For example, Japanese Unexamined Patent Publication No. 1999-503262 discloses phosphoric acid-doped polybenzimidazole and like materials as examples of unhumidified polymer electrolytic membranes. However, in such unhumidified polymer electrolytic membranes, phosphoric acid or a like strong acid leaks with long-term operation, the open-circuit voltage lowers, and the cell resistance increases, which may adversely affect operating stability.
In order to solve such problems, a technique wherein a carbon layer is formed between an electrolytic membrane and a catalytic layer to retain electrolyte and prevent the transmission of a reaction gas, so that open-circuit voltage and cell resistance can be shifted in a stable manner (Japanese Unexamined Patent Publication No. 2006-32275).
In the technique disclosed in Japanese Unexamined Patent Publication No. 2006-32275, the carbon layer is made thicker than the electrolytic membrane in order to stabilize the solid polymer electrolytic membrane without increasing cell resistance (paragraph 0031 of Japanese Unexamined Patent Publication No. 2006-32275). To maintain the ion conductivity of the solid polymer electrolytic membrane for a long time, the carbon layer is additionally impregnated with a strong acid (paragraph 0030 of Japanese Unexamined Patent Publication No. 2006-32275). However, if the carbon layer is made thicker than the electrolytic membrane, the proton conductivity will be decreased. Furthermore, by additionally impregnating the carbon layer with a strong acid, gas diffusion is adversely affected by excessive acid. Therefore, the technique of Japanese Unexamined Patent Publication No. 2006-32275 has not yet been brought into practical use.